Inset: Walter Mousseau Jr. (Pennington County Jail). Background: The park where Mousseau drowned a man in Rapid City, S.D. (Rapid City Parks and Recreation).
A South Dakota man will spend the rest of his life behind bars for drowning another man in broad daylight as horrified onlookers watched the victim slowly dying before their eyes.
In November 2025, Walter Mousseau Jr., 30, was found guilty but mentally ill on one count of murder in the second degree.
On New Year”s Eve 2025, he was given a mandatory life sentence by 7th Judicial Circuit Court Judge Eric Kelderman.
The punishment stems from the murder of Sheldon Glenn, 43, who Mousseau drowned at a pond amid festivities on July 4, 2022. The fatal incident occurred at Memorial Park in Rapid City – a medium-sized city located some 20 miles northeast of Mount Rushmore.
The since-condemned man’s factual culpability was never really in doubt. His defense claimed he was not guilty by reason of insanity during the first day of his trial, which was delayed for years over mental competency reasons, according to the Rapid City Journal.
After two weeks of testimony from witnesses, law enforcement, and mental health experts, the jury sided with the state and rejected the insanity defense, according to a courtroom report by the Journal.
“When a defendant is found legally insane, the focus shifts entirely to psychiatric treatment aimed at addressing the danger posed to society, and the law requires release once the individual is no longer considered a danger to society,” a spokesperson for the Pennington County State’s Attorney’s Office told the paper.
Embedded within the rejected insanity claim was the idea that the defendant’s “warped perception of reality” led him to genuinely believe he was acting in self-defense on the day he killed Glenn.
The state called this idea “fundamentally wrong.”
During the trial, witnesses were called to recall how onlookers begged Mousseau to let up and pull the victim to shore. Later, Mousseau told police he rejected those pleas for mercy as the other man slowly died because he did not want to “seem weak.”
“He was aware of what was happening around him, what he was doing,” prosecutor Emily Toms said. “But he chose to ignore it. The insanity defense protects those who can’t comprehend what they are doing. The defendant did, and he decided to kill Sheldon Glenn.”
Still, the defense argued there was more to it than that.
Psychiatrists testified about the defendant’s schizophrenia and a history of methamphetamine use as mitigating factors for the violence and compounding factors for the mental health argument. The way Mousseau saw the world was also explored as jurors heard he believed there were eyes in the sky that day and that Glenn could be brought back to life if only the water were removed from his body.
During sentencing, the state again rejected mental illness as an excuse.
“This was a choice to escalate violence to its highest and most irreversible level,” prosecutor Jason Thomas said, according to the Journal.
In fact, Mousseau also told police he had long intended to kill someone in self-defense if given the opportunity, the prosecutor added. He said the case was an example of a “broadly and deeply troubling pattern” of cases where people feel justified in killing one another, “based on perceived slights, disrespect, or anger.”
“Self-defense is not a license to kill,” Thomas said.
By the time of sentencing, due to Mount Rushmore State law, Mousseau’s sentencing was not really much in doubt either.
The prosecution, for its part, said the sentencing hearing was an important and necessary process for the victim’s loved ones.
“This case is about the value of a human life, Sheldon Glenn’s life,” Thomas said in a press release. “Sheldon was a person, not a name in a case file. He had relationships, history, and a future that existed until the moment the defendant decided it no longer did. That opportunity was taken away when the defendant chose to escalate a minor conflict into deadly violence. What was lost that day cannot be restored, and it will never be replaced.”
